From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Effects of different tyre sizes and axle types on the 20 HP model's overall gear ratio.
| Identifier | ExFiles\Box 102\4\ scan0029 | |
| Date | 9th May 1928 | |
| To CWB. from Hs.{Lord Ernest Hives - Chair} X5350 Hs{Lord Ernest Hives - Chair}1/LG19.5.28. 20 HP. AXLE RATIOS. X5350 X5820 The effective rolling circumferences of the 31 X 5.25 and 31 X 6" tyres at 35 lbs/sq.in. are :- 31 X 6 100" 31 X 5.25 97.5" However, due to its larger section, the 6" tyre can, without danger, be run at 5 lbs/sq.in. lower pressure when its effective circumference becomes 99.5". Therefore we may say that at present the gear ratio of the car will be altered 2% by fitting the 6" tyre. The old 11 X 50 axle gives a ratio of 4.55 - 1 The new 10 X 47 " " " " " 4.7 - 1 Therefore with the new axle the gear ratio will be lowered 3.3%, or by fitting the 10 X 47 axle and 6" tyres the gear ratio will be 1.3% lower than the present 11 X 50 axle with 5.25" tyres. Hs.{Lord Ernest Hives - Chair} | ||